Porous electrode active material and secondary battery including the same

ABSTRACT

Provided are an electrode active material having a plurality of pores and a secondary battery including the same, and more particularly, a porous electrode active material including silicon-based oxide expressed by SiO x  (0.5≦x≦1.2) and having a Brunauer, Emmett, and Teller (BET) specific surface area ranging from 2 m 2 /g to 100 m 2 /g, and a secondary battery including a cathode including a cathode active material, a separator, an anode including an anode active material, and an electrolyte, in which the anode active material includes a porous electrode active material including silicon-based oxide expressed by SiO x  (0.5≦x≦1.2) and having a BET specific surface area ranging from 2 m 2 /g to 100 m 2 /g.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 13/566,553, filed on Aug. 3, 2012, and claims the priority ofKorean Patent Application No. 10-2012-0041083 filed on Apr. 19, 2012,and Korean Patent Application No. 10-2012-0076953, filed on Jul. 13,2012, in the Korean Intellectual Property Office, the disclosures ofwhich are incorporated herein by reference.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a porous electrode active material anda secondary battery including the same.

Description of the Related Art

Lithium secondary batteries recently have received most attention due totheir high energy density and long lifetime. Typically, a lithiumsecondary battery includes an anode formed of a carbon material orlithium metal alloy, a cathode formed of lithium metal oxide, and anelectrolyte having a lithium salt dissolved in an organic solvent.

Lithium metal is initially used as an anode active material constitutingan anode electrode of a lithium secondary battery. However, sincelithium may have low reversibility and safety, a carbon material iscurrently mainly used as the anode active material of the lithiumsecondary battery. The carbon material may have capacity lower than thatof metal, but the carbon material may have low changes in volume andexcellent reversibility, and may be favorable in terms of price.

However, demand for high-capacity lithium secondary batteries hasgradually increased as the use of lithium secondary batteries has beenincreased. Accordingly, a high-capacity electrode active materialcapable of substituting the carbon material having low capacity isrequired. For this purpose, research into using metal (metalloid)exhibiting charge and discharge capacity higher than that of the carbonmaterial and electrochemically alloyable with lithium, e.g., silicon(Si) and tin (Sn), as an electrode active material has been conducted.

However, the metal (metalloid)-based electrode active material has highchanges in volume accompanying charge and discharge of lithium and thus,cracks and pulverization may be generated. Therefore, capacity of thebattery may rapidly decrease and cycle lifetime may decrease as chargeand discharge cycles are performed.

SUMMARY OF THE INVENTION

An aspect of the present invention provides an electrode active materialfor a secondary battery able to prevent generation of cracks andpulverization through reducing volume changes despite of using metal(metalloid) oxide, such as silica, as an electrode active material aswell as having improved lifetime characteristics and a low thicknesschange rate.

According to an aspect of the present invention, there is provided aporous electrode active material including silicon-based oxide expressedby SiO_(x) (0.5≦x≦1.2) and having a Brunauer, Emmett, and Teller (BET)specific surface area ranging from 2 m²/g to 100 m²/g.

According to another aspect of the present invention, there is provideda secondary battery including a cathode including a cathode activematerial; a separator; an anode including an anode active material; andan electrolyte, wherein the anode active material includes a porouselectrode active material including silicon-based oxide expressed bySiO_(x) (0.5≦x≦1.2) and having a BET specific surface area ranging from2 m²/g to 100 m²/g.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a scanning electron microscope (SEM) photograph of porous SiOaccording to an embodiment of the present invention; and

FIG. 2 is a graph showing results of X-ray diffraction (XRD) analysis onporous SiO according to the embodiment of the present invention andtypical SiO.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides a porous electrode active materialincluding silicon-based oxide expressed by SiO (0.5≦x≦1.2) and having aBrunauer, Emmett, and Teller (BET) specific surface area ranging from 2m²/g to 100 m²/g.

Hereinafter, the present invention will be described in detail.

An electrode active material according to an embodiment of the presentinvention includes pores and siliconbased oxide expressed by SiO_(x)(0.5≦x≦1.2). More particularly, the electrode active material may have aBET specific surface area ranging from 2 m²/g to 100 m²/g, may include aporosity ranging from 5% to 90% of a total volume of the electrodeactive material, and is particles having pores in a honeycomb shapeformed at least on surfaces thereof. In the case that the BET specificsurface area or porosity of the electrode active material is less than alower limit value, volume expansion of SiO_(x) during charge anddischarge may not be prevented, and in the case that the BET specificsurface area or porosity of the electrode active material is greaterthan an upper limit value, mechanical strength may be decreased due to alarge amount of pores existing in SiO_(x) and thus, SiO_(x) may bedestroyed during preparation processes (slurry mixing, pressing aftercoating, etc.) of a battery.

Also, in the electrode active material according to the embodiment ofthe present invention, x in SiO_(x) may be in a range of 0.5 to 1.2 andx may be 1. In the case that x is less than 0.5, initial efficiency maybe high, but an amount of oxygen able to inhibit volume expansion may below, and thus, lifetime and inhibition of thickness expansion maydecrease despite of forming a porous structure. In the case that x isgreater than 1.2, the initial efficiency may decrease due to an increasein the amount of oxygen.

The electrode active material according to the embodiment of the presentinvention may be coated with carbon in order to improve batteryperformance of a secondary battery.

In the case of using the silicon-based oxide according to the embodimentof the present invention as an electrode active material, anirreversible phase, such as lithium oxide or lithium silicon oxide, isformed due to a reaction between lithium ions (Li⁺) inserted into ananode and silicon-based oxide during initial charge and discharge of abattery, and at this time, since the irreversible phase surroundssilicon in the silicon oxide, less cracks or pulverization aregenerated. Also, since pores exist at least on the surface or both thesurface and the inside of the electrode active material, capacity of thebattery may be improved and volume changes generated during charge anddischarge may be effectively controlled, and thus, improved lifetime maybe obtained. The electrode active material according to the embodimentof the present invention may be used as both cathode active material andanode active material, but may be an anode active material.

Further, the present invention provides a method of preparing anelectrode active material including: mixing a fluorine-based solutionand a metal precursor solution and then allowing SiO_(x)(0.5≦x≦1.2)-containing particles to be in contact therewith toelectrodeposit metal particles on surfaces of the SiO_(x)-containingparticles; etching the SiO_(x)-containing particles by allowing theSiO_(x)-containing particles having metal particles electrodepositedthereon to be in contact with an etching solution; and removing themetal particles by allowing the etched SiO_(x)-containing particles tobe in contact with a metal removing solution.

The method of preparing an electrode active material according to anembodiment of the present invention may form pores without changing acrystal structure of the SiO_(x) (0.5≦x≦1.2)-containing particles.

The method of preparing an electrode active material according to theembodiment of the present invention includes mixing a fluorine-basedsolution and a metal precursor solution and then allowingSiO_(x)-containing particles to be in contact therewith toelectrodeposit metal particles in the metal precursor solution on theSiO_(x)-containing particles. At this time, the SiO_(x)-containingparticles emit electrons due to the fluorine-based solution and metalions in the solution receive electrons to be reduced andelectrodeposited on the surfaces of the SiO_(x)-containing particles.Once the metal particles are electrodeposited on the surface of theSiO_(x)-containing particles, continuous electrodeposition may begenerated as the metal particle itself becomes a catalyst site.

The fluorine-based solution used may be one or more selected from thegroup consisting of hydrogen fluoride (HF), silicon fluoride (H₂SiF₆),and ammonium fluoride (NH₄F), and the metal precursor solution mayinclude one or more selected from the group consisting of silver (Ag),gold (Au), platinum (Pt), and copper (Cu). The fluorine-based solutionand the metal precursor solution may be mixed at a volume ratio rangingfrom 10:90 to 90:10. In the case that the volume ratio of thefluorine-based solution included is less than 10, an amount of the metalprecursor formed on the surfaces of the SiO_(x)-containing particles maybe small and a reaction rate may be very slow, and thus, a preparationtime may increase. In the case that the volume ratio of thefluorine-based solution included is greater than 90, formation speed ofthe metal precursor may be very fast, and thus, uniform and small-sizedmetal particles may not be electrodeposited on the surfaces of theSiO_(x)-containing particles.

Also, an amount of the metal particles electrodeposited on theSiO_(x)-containing particles may be controlled according to aconcentration of the fluorine-based solution and a contact time of theSiO_(x)-containing particles with the metal precursor solution. Acontent of the contacted SiO_(x)-containing particles may be in a rangeof 0.001 to 50 parts by weight based on 100 parts by weight of a mixedsolution of the fluorine-based solution and the metal precursorsolution.

The method of preparing an electrode active material according to theembodiment of the present invention includes etching theSiO_(x)-containing particles by allowing the SiO_(x)-containingparticles having metal particles electrodeposited thereon to be incontact with an etching solution. Nanopores, mesopores, and macroporesare formed in the SiO_(x)-containing particles through the etchingprocess.

Metal particles are oxidized by H₂O₂ and become metal ions, theSiO_(x)-containing particles are continuously dissolved while electronsare transferred from the SiO_(x)-containing particles to the metalparticles at interfaces between the SiO_(x)-containing particles and themetal particles, and reduction of metal ions oxidized from the metalparticles electrodeposited on the surfaces of the foregoingSiO_(x)-containing particles is generated. According to the foregoingmethod, the SiO_(x)-containing particles in contact with the metalparticles may be continuously etched to form a porous structure having ahoneycomb shape at least on the surface thereof, and a size of the metalparticles may increase because the metal particles have a strongtendency to agglomerate with adjacent metal particles in the etchingsolution during etching.

A mixed solution of hydrogen fluoride (HF) solution and hydrogenperoxide (H₂O₂) solution may be used as the etching solution and anamount of the hydrogen fluoride solution may vary according to a degreeof etching. However, the hydrogen fluoride solution and the hydrogenperoxide solution may be mixed at a volume ratio ranging from 10:90 to90:10. At this time, a content of H₂O₂ plays an important role informing mesopores in the SiO_(x)-containing particles and an oxidizedamount of the metal particles is determined by a concentration of H₂O₂and thus, a concentration of the metal ions may be determined. The metalparticles become metal ions by H₂O₂, the metal ions begin to adhere tospecific defective sites (e.g., portions having SiO_(x) etchedtherefrom), and mesopores are formed by etching under theSiO_(x)-containing particles having metal adhered thereto.

Also, the etching may be performed for 30 minutes to 5 hours. In thecase that the etching is performed less than 30 minutes, the formationof pores in the SiO_(x)-containing particles may be insignificant, andin the case that the etching is performed greater than 5 hours, theSiO_(x)-containing particles are excessively etched and thus, mechanicalproperties of the SiO_(x)-containing particles may be degraded.

The method of preparing an electrode active material according to theembodiment of the present invention includes removing the metalparticles by allowing the etched SiO_(x)-containing particles to be incontact with a metal removing solution, and may prepare particles havingpores in a honeycomb shape formed at least on the surface of theSiO_(x)-containing particles.

The metal removing solution used may be one or more selected from thegroup consisting of nitric acid (HNO₃), sulfuric acid (H₂SO₄), andhydrochloric acid (HCl).

The electrode active material prepared according to the preparationmethod of the present invention may be used as both cathode activematerial and anode active material, but may be used as an anode activematerial.

Also, the present invention provides a secondary battery including acathode including a cathode active material; a separator; an anodeincluding an anode active material; and an electrolyte, in which theanode active material includes a porous electrode active materialincluding silicon-based oxide expressed by SiO_(x) (0.5≦x≦1.2) andhaving a BET specific surface area ranging from 2 m²/g to 100 m²/g.

At this time, the anode active material may have a porosity ranging from5% to 90%, may include pores having a honeycomb shape at least on thesurface thereof, and the SiO_(x) may be silicon monoxide in which x is1.

The anode active material according to the embodiment of the presentinvention may be used in a secondary battery by mixing with a typicallyused anode active material, and the typically used anode active materialmay be one or more selected from the group consisting of graphite, softcarbon, hard carbon, and lithium titanium oxide.

The prepared electrode active material, specifically an anode activematerial, may be prepared as an anode by using a preparation methodtypically used in the art. For example, the anode active material of thepresent invention is mixed and stirred with a binder, a solvent, and aconductive material and a dispersant if necessary to prepare slurry, andthen an anode may be prepared by coating a collector with the slurry andpressing.

Examples of the binder may be a vinylidene fluoride-hexafluoropropylenecopolymer (PVDF-co-HEP), polyvinylidene fluoride, polyacrylonitrile,polymethylmethacrylate, polyvinyl alcohol, carboxymethyl cellulose(CMC), starch, hydroxypropyl cellulose, regenerated cellulose,polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene,polyacrylate, a ethylene-propylene-diene monomer (EPDM), a sulfonatedEPDM, a styrene-butadiene rubber (SBR), a fluoro rubber, and variouscopolymers.

N-methyl-2-pyrrolidone, acetone, or water may be used as the solvent.

The conductive material is not particularly limited so long as it doesnot cause chemical changes in the battery and has conductivity. Forexample, graphite such as natural graphite or artificial graphite;carbon black such as acetylene black, Ketjen black, channel black,furnace black, lamp black, and thermal black; conductive fibers such ascarbon fibers or metal fibers; metal powders such as fluoro carbonpowder, aluminum powder, and nickel powder; conductive whiskers such aszinc oxide whiskers and potassium titanate whiskers; conductive metaloxides such as titanium oxide; and conductive materials such aspolyphenylene derivatives may be used as the conductive material.

An aqueous-based dispersant or an organic dispersant such asN-methyl-2-pyrrolidone may be used as the dispersant.

Similar to the preparation of the foregoing anode, a cathode activematerial, a conductive material, a binder, and a solvent are mixed toprepare a slurry, and then a cathode may be prepared by directly coatinga metal collector with the slurry or by casting the slurry on a separatesupport and laminating a cathode active material film separated from thesupport on a metal collector.

Examples of the cathode active material may be a layered compound, suchas lithium cobalt oxide (LiCoO₂) or lithium nickel oxide (LiNiO₂), or acompound substituted with one or more transition metals; lithiummanganese oxides such as Li_(1+y)Mn_(2−y)O₄ (where y is 0 to 0.33),LiMnO₃, LiMn₂O₃, and LiMnO₂; lithium copper oxide (Li₂CuO₂); vanadiumoxides such as LiV₃O₈, LiFe₃O₄, V₂O₅, and Cu₂V₂O₇; nickel (Ni)-site typelithium nickel oxide expressed by a chemical formula ofLiNi_(1−y)M_(y)O₂ (where M is cobalt (Co), manganese (Mn), aluminum(Al), copper (Cu), iron (Fe), magnesium (Mg), boron (B), or gallium(Ga), and y is 0.01 to 0.3); lithium manganese complex oxide expressedby a chemical formula of LiMn_(2−y)M_(y)O₂ (where M is Co, Ni, Fe,chromium (Cr), zinc (Zn), or tantalum (Ta), and y is 0.01 to 0.1) orLi₂Mn₃MO₈ (where M is Fe, Co, Ni, Cu, or Zn); LiMn₂O₄ having a part oflithium (Li) being substituted with an alkaline earth metal ion; adisulfide compound; and Fe₂(MoO₄)₃. However, the cathode active materialis not limited thereto.

A typical porous polymer film used as a typical separator, for example,a porous polymer film prepared from a polyolefin-based polymer, such asan ethylene homopolymer, a propylene homopolymer, an ethylene/butenecopolymer, an ethylene/hexene copolymer, and an ethylene/methacrylatecopolymer, may be used alone or in a lamination therewith as theseparator. A typical porous nonwoven fabric, for example, a nonwovenfabric formed of high melting point glass fibers or polyethyleneterephthalate fibers may be used, but the separator is not limitedthereto.

In an electrolyte solution used in the present invention, a lithiumsalt, which may be included as an electrolyte, may be used withoutlimitation so long as it is typically used in an electrolyte solutionfor a secondary battery. For example, one selected from the groupconsisting of F⁻, Cl⁻, I⁻, NO₃ ⁻, N(CN)₂ ⁻, BF₄ ⁻, ClO₄ ⁻, PF₆ ⁻,(CF₃)₂PF₄ ⁻, (CF₃)₃PF₃ ⁻, (CF₃)₄PF₂ ⁻, (CF₃)₅PF⁻, (CF₃)₆P⁻, CF₃SO₃ ⁻,CF₃CF₂SO₃ ⁻, (CF₃SO₂)₂N⁻, (FSO₂)₂N⁻, CF₃CF₂(CF₃)₂CO⁻, (CF₃SO₂)₂CH⁻,(SF₅)₃C⁻, (CF₃SO₂)₃C⁻, CF₃ (CF₂)₇SO₃ ⁻, CF₃CO₂ ⁻, CH₃CO₂ ⁻, SCN⁻, and(CF₃CF₂SO₂)₂N⁻ may be used as an anion of the lithium salt.

In the electrolyte solution used in the present invention, an organicsolvent included in the electrolyte solution may be used withoutlimitation so long as it is typically used, and typically, one or moreselected from the group consisting of propylene carbonate, ethylenecarbonate, diethyl carbonate, dimethyl carbonate, ethylmethyl carbonate,methylpropyl carbonate, dipropyl carbonate, dimethyl sulfoxide,acetonitrile, dimethoxyethane, diethoxyethane, vinylene carbonate,sulfolane, γ-butyrolactone, propylene sulfite, and tetrahydrofuran maybe used.

In particular, ethylene carbonate and propylene carbonate, ring-typecarbonates among the carbonate-based organic solvents, well dissociatethe lithium salt in the electrolyte solution due to high dielectricconstants as high-viscosity organic solvents, and thus, the ring-typecarbonate may be used. Since an electrolyte solution having highelectrical conductivity may be prepared when the ring-type carbonate ismixed with low-viscosity, low-dielectric constant linear carbonate, suchas dimethyl carbonate and diethyl carbonate, in an appropriate ratio,and thus, the ring-type carbonate, for example, may be used.

Selectively, the electrolyte solution stored according to the presentinvention may further include an additive, such as an overchargeinhibitor, included in a typical electrolyte solution.

A separator is disposed between the cathode and the anode to form abattery structure, the battery structure is wound or folded to put in acylindrical battery case or prismatic battery case, and then a secondarybattery is completed when the electrolyte is injected thereinto. Also,the battery structure is stacked in a bi-cell structure and thenimpregnated with the electrolyte solution, and a secondary battery iscompleted when the product thus obtained is put in a pouch and sealed.

Hereinafter, the present invention will be described in detail accordingto specific examples. The invention may, however, be embodied in manydifferent forms and should not be construed as being limited to theembodiments set forth herein.

EXAMPLE 1 Preparation of Porous SiO 1

1. Electrodeposition of Ag on Surfaces of SiO Particles

300 ml of a solution having 10% of hydrogen fluoride (HF) and 300 ml ofa solution having 10 mM of silver nitrate (AgNO₃) were mixed for 10minutes. 2g of silicon monoxide (SiO) was added to the solution havinghydrogen fluoride and silver nitrate mixed therein and the solution wasmixed for 5 minutes, and then SiO having silver (Ag) electrodepositedthereon was prepared by filtering, washing, and drying the mixture.

2. Chemical Etching

200 ml of a solution having 5% of hydrogen fluoride and 100 ml of asolution having 1.5 wt% of hydrogen peroxide (H₂O₂) added therein weremixed for 10 minutes. SiO having Ag particles electrodeposited thereonwas added to the etching solution having hydrogen fluoride and hydrogenperoxide mixed therein and mixed for 30 minutes, and then porous SiO wasprepared by filtering, washing, and drying the mixture.

3. Ag Removal

100 ml of 60% nitric acid (HNO3) was heated to 50° C. and the porous SiOwas then added thereto and mixed for 2 hours. A porous SiO for an anodeactive material having Ag removed therefrom was prepared by filtering,washing, and drying the mixture.

EXAMPLE 2 Preparation of Porous SiO 2

An anode active material was prepared in the same manner as Example 1except that SiO having Ag electrodeposited thereon was added to theetching solution having hydrogen fluoride and hydrogen peroxide mixedtherein and mixed for 2 hours.

EXAMPLE 3 Preparation of Porous SiO 3

An anode active material was prepared in the same manner as Example 1except that SiO having Ag electrodeposited thereon was added to theetching solution having hydrogen fluoride and hydrogen peroxide mixedtherein and mixed for 5 hours.

EXAMPLE 4 Preparation of Secondary Battery 1

The porous SiO prepared in Example 1 as an anode active material,acetylene black as a conductive material, and polyvinylidene fluoride asa binder were mixed at a weight ratio of 88:2:10 and the mixture wasmixed with a N-methyl-2-pyrrolidone solvent to prepare a slurry. Onesurface of a copper collector was coated with the prepared slurry to athickness of 65 μm, dried and rolled, and then an anode was prepared bypunching into a predetermined size.

LiPF₆ was added to a non-aqueous electrolyte solvent prepared by mixingethylene carbonate and diethyl carbonate at a volume ratio of 30:70 toprepare a 1 M LiPF₆ non-aqueous electrolyte solution.

A lithium foil was used as a counter electrode and a polyolefinseparator was disposed between both electrodes, and then a coin typesecondary battery was prepared by injecting the electrolyte solution.

EXAMPLE 5 Preparation of Secondary Battery 2

A coin type secondary battery was prepared in the same manner as Example4 except that the porous SiO prepared in Example 2 was used as an anodeactive material.

EXAMPLE 6 Preparation of Secondary Battery 3

A coin type secondary battery was prepared in the same manner as Example4 except that the porous SiO prepared in Example 3 was used as an anodeactive material.

COMPARATIVE EXAMPLE 1

A coin type secondary battery was prepared in the same manner as Example4 except that non-porous SiO (bare SiO) was used as an anode activematerial.

COMPARATIVE EXAMPLE 2

A coin type secondary battery was prepared in the same manner as Example4 except that Si having a porosity of 13% was used.

COMPARATIVE EXAMPLE 3

A coin type secondary battery was prepared in the same manner as Example4 except that Si having a porosity of 35% was used.

COMPARATIVE EXAMPLE 4

A coin type secondary battery was prepared in the same manner as Example4 except that Si having a porosity of 75% was used.

EXPERIMENTAL EXAMPLE 1 Analysis on Shape and Crystal Structure of PorousSiO

Scanning electron microscope (SEM) and X-ray diffraction (XRD) were usedin order to analyze shape and crystal structure of porous SiO accordingto the embodiment of the present invention, and the results thereof arepresented in FIGS. 1 and 2.

As shown in FIG. 1, it may be confirmed that a plurality of pores in ahoneycomb shape was formed on the surface and the inside of the porousSiO (Example 2) according to the embodiment of the present invention(FIG. 1(b) is an enlarged view of FIG. 1(a)).

Also, as shown in FIG. 2, it may be understood that the porous SiO(Example 2) according to the embodiment of the present invention and SiO(bare SiO) had the same crystal structure, and accordingly, it may beunderstood that the porous SiO of the present invention has the samecomposition and crystal structure as those of raw material SiO and Agwas completely removed therefrom.

EXPERIMENTAL EXAMPLE 2 Evaluation of Porosity, BET Specific SurfaceArea, Lifetime Characteristics, and Thickness Change Rate of SecondaryBattery

The following experiments were performed in order to investigateporosities, Brunauer, Emmett, and Teller (BET) specific surface areas,lifetime characteristics, and thickness change rates of the coin typesecondary batteries prepared in Examples 4 to 6 and Comparative Examples1 to 4.

In order to investigate charge and discharge characteristics of thesecondary batteries, coin type secondary batteries prepared in Examples4 to 6 and Comparative Examples 1 to 4 were charged to 5 mV at aconstant current, and then charged until the current reaches 0.005 C at5 mV and the charging was terminated. Discharge of the batteries wasperformed to 1.0 V at a constant current.

BET specific surface areas were calculated by allowing nitrogen (N₂) tobe adsorbed on surfaces of the porous SiO of Examples 1 to 3 and the Siof Comparative Examples 1 to 4 and measuring amounts of adsorbednitrogen gas. Charge and discharge were performed at 0.5 C after a thirdcycle and lifetime characteristics of the battery were calculated bymeasuring a ratio of discharge capacity of a 49th cycle to dischargecapacity of a first cycle.

Each coin type secondary battery was disassembled in a charge state of a50th cycle and a thickness change rate was calculated by measuringdifference in thicknesses of an electrode after the 50th cycle andbefore a charge cycle.

The following Table 1 presents porosities, discharge capacities, initialefficiencies, lifetime characteristics, and thickness change rates ofthe coin type secondary batteries prepared in Examples 4 to 6 andComparative Examples 1 to 4.

TABLE 1 Lifetime Thickness Porosity BET characteristics change rateExamples (%) (m²/g) (%) (%) Example 4 11 12.5 88 157 Example 5 38 42.896 86 Example 6 72 98.3 99 43 Comparative 0 2.1 85 194 Example 1Comparative 13 14.3 32 653 Example 2 Comparative 35 37.1 42 580 Example3 Comparative 75 99.5 67 427 Example 4

-   -   Lifetime characteristics: (discharge capacity of the 49th        cycle/discharge capacity of the first cycle)×100    -   Thickness change rate: (electrode thickness after the 50th        cycle−electrode thickness before a cycle)/electrode thickness        before a cycle×100

As shown in Table 1, it may be understood that lifetime characteristicsof the batteries prepared in Examples 4 to 6 according to the presentinvention were improved from a minimum of 3% to a maximum of 67% incomparison to those of the batteries prepared in Comparative Examples 1to 4, and it may be also understood that the difference in thicknesschange rates ranged from a minimum of 37% to a maximum of 610%. It maybe understood that the electrode active material according to thepresent invention, different from typical Si, included oxygen and aplurality of pores, and thus, lifetime characteristics and thicknesschange rate were greatly improved.

In the present invention, since pores are formed on surface and insideof silicon-based oxide by physically controlling a crystal structurewithout changing the crystal structure, a secondary battery not only hashigh capacity but volume changes generated during charge and dischargeare also effectively controlled. Therefore, volume changes are small andlifetime characteristics are excellent.

While the present invention has been shown and described in connectionwith the exemplary embodiments, it will be apparent to those skilled inthe art that modifications and variations can be made without departingfrom the spirit and scope of the invention as defined by the appendedclaims.

What is claimed is:
 1. A method of preparing a porous electrode active material comprising silicon-based oxide expressed by SiO_(x) (0.5≦x≦1.2) and having a Brunauer, Emmett, and Teller (BET) specific surface area ranging from 42.8 m²/g to 100 m²/g, wherein porosity is in a range of from 38% to 90%, and wherein pores in a honeycomb shape are included at least on a surface of the porous electrode active material, the method comprising: electrodepositing metal particles on surfaces of the SiO_(x) (0.5≦x≦1.2) particles in a mixed solution of a fluorine-based solution and a metal precursor solution, wherein an amount of the SiO_(x) particles is present in a range of 0.001 to 50 parts by weight based on 100 parts by weight of the mixed solution; etching the surface of the SiO_(x) particles under the metal particles electrodeposited thereon in an etching solution for a period ranging from 2 hours to 5 hours, wherein the etching solution includes hydrogen fluoride (HF) and hydrogen peroxide (H₂O₂) mixed at a volume ratio ranging from 10:90 to 90:10; and removing the metal particles from the etched SiO_(x) particles in a metal removing solution.
 2. The method of claim 1, wherein the fluorine-based solution includes one or more selected from the group consisting of hydrogen fluoride (HF), silicon fluoride (H₂SiF₆), and ammonium fluoride (NH₄F).
 3. The method of claim 1, wherein the metal precursor solution comprises one or more selected from the group consisting of silver (Ag), gold (Au), platinum (Pt), and copper (Cu).
 4. The method of claim 1, wherein the fluorine-based solution and the metal precursor solution are present in the mixed solution at a volume ratio ranging from 10:90 to 90:10.
 5. The method of claim 1, wherein the metal removing solution is one or more selected from the group consisting of nitric acid (HNO₃), sulfuric acid (H₂SO₄), and hydrochloric acid (HCl). 